The recently discovered apelin family of peptides mediate their actions by a single G-protein coupled receptor, APJ. We have previously shown that apelins have three major actions in the human cardiovascular system: endothelium-dependent vasodilatation; direct vasoconstriction by interacting with smooth muscle APJ receptor and increased cardiac contractility by action on cardiac myocytes. Our aim was to discover shorter sequences of (Pyr1)apelin-13 retaining agonist activity and use a computational ligand-based strategy to design cyclic peptide agonists and antagonists. Over 50 compounds were synthesised and tested in a competition binding assay. Functional assays for agonists measured vasoconstrictor action in endothelium-denuded human saphenous veins. Antagonist activity was measured in cyclic AMP assays against (Pyr1)apelin-13. Data are expressed as mean±SE, pD2=−log10EC50, Emax=maximum response. The predicted cleavage product of the angiotensin converting enzyme 2 (ACE-2), apelin-13(1–12), inhibited radiolabelled apelin binding and was identified as the shortest sequence potently constricting endothelium-denuded saphenous vein (pD2 9.07%±0.40, Emax 29.30%±9.43% KCl, n=5). The most potent cyclic analogue identified, MM07, inhibited binding with a KD=86%±30 nM, (n=3) and pD2 10.53%±0.24, Emax 21.80%±5.72% KCl, n=3) with a comparable potency and efficacy to (Pyr1)apelin-13 (pD2=8.8%±0.3, Emax 26%±4%, n=15). MM54 inhibited binding, KD=3.42%±0.45 μmol/l, (n=3) and was identified as an antagonist with a pA2 value of 5.9 in a cyclase inhibition assay. The novel cyclic peptide MM07 retains potency and we have identified the first apelin receptor antagonist, MM54 as a pharmacological tool to characterise the apelin system and in the design of small molecule drugs.